This invention relates to compositions for refrigeration systems. More particularly, the compositions contain one or a mixture of esters of alkane polycarboxylic acid esters which are useful as synthetic lubricants in compositions containing fluorine-containing hydrocarbons.
Chlorofluorocarbons, generally referred to in the industry as CFCs, have been widely used in refrigeration systems. The use of CFCs has been diminishing in recent years because of the detrimental effect on the earth's ozone layer that CFCs are believed to cause. Finding a safe, efficient replacement for CFC's has been a problem which is difficult to solve. Several replacement candidates have been suggested as alternatives to the fully halogenated hydrocarbons. Examples of alternatives include halogenated hydrocarbons containing at least one hydrogen atom such as HCFC-22 which is difluorochloromethane, HCFC-123 which is 1,1-dichloro-2,2,2-trifluoroethane, HFC-134a which is 1,1,1,2-tetrafluoroethane, and HCFC-141b which is 1,1-dichloro-1-fluoroethane. Of the foregoing, HCFC-22 and HFC-134a are generally recommended as being candidates in refrigerant applications, with HFC-134a being particularly attractive because its ozone depletion potential is believed to be zero and its thermodynamic properties are similar to those of CFC 12 (dichlorodifluoromethane).
One problem with using alternative refrigerant materials is that the alternative materials have different solubility characteristics than the CFCs used in refrigerants presently. For example, the mineral lubricating oil is incompatible (i.e., insoluble) in HFC-134a. Such incompatibility can result in unacceptable compressor life in compressor type refrigeration equipment including commercial, home and industrial refrigerators and air-conditioners for auto, home, and industrial use.
Numerous refrigeration lubrication oils have been proposed for use with HFC-134a; however, such oils have advantages and disadvantages when taking into account the following typical properties required in service:
(1) Optimum viscosity PA0 (2) Good low temperature characteristics including low pour point and low floc point. PA0 (3) Low critical temperature for dissolution in refrigerants. PA0 (4) Good data on sealed-tube tests; that is, no discoloration, no precipitation and no copper plating. PA0 (5) Good thermal and chemical stability even in the presence of halogen-containing refrigerants.
In addition to the foregoing criteria for the value of refrigeration lubricants, there is another important factor which must be considered, particularly where the lubrication oil is intended to be used in refrigerators operating on a compressor system. It is the capability of a given oil to establish a gas seal between the low pressure side (suction) and the high pressure side (discharge) of the compressor. Modern refrigeration compressors have a tendency to encounter increased gas temperatures at the discharge side, which would lead to reduced oil viscosity and hence to loss in the sealability.
A recent paper by K. S. Sanvordenker, "Materials Compatibility of HFC-134a in Refrigerant Systems", pps. 211-216 (presented at the ASHRE Winter Meeting in January 1989) discusses several proposed refrigerant lubricants One proposed lubricant is a polyglycol lubricant, e.g., polypropylene glycol monoether, polyethylene-propylene glycol monoether, etc. Polyoxyalkylene glycol monoethers are disclosed as refrigerant lubricants in U.S. Pat. Nos. 4,267,064, 4,755,316, and 4,948,525. The miscibilities of polyglycols with HFC-134a indicate that the immiscibility region is at high temperatures. U.S. 4,851,144 relates to the use of a polyether polyol as a lubricant in a composition with HFC-134a refrigerant. Another proposed lubricant is a dibasic acid ester or a neopentyl ester.
Polyalkylene glycols, esters, and amides all appear to be useful as lubricants according to Scott T. Jolley's paper, "New and Unique Lubricants for Use in Compressors Utilizing R-134a Refrigerant", pps. 145-152 (given at the ASHRE/Refrigeration/Compressor Engineering Conference at Purdue, July 1990). This paper observes that of the three, ester-type base fluids seem to offer the greatest number of advantages, followed by the polyalkyleneglycols and amides. Tertiary amides of the formula ##STR1## are disclosed in U.S. 4,992,188 as being suitable for use with HFC-134a refrigerants.
In WO 90/06979 (U.S. Ser. No. 284,315), a liquid composition is described which comprises: (A) a major amount of a fluorine containing hydrocarbon containing one or two carbon atoms, and (B) a minor amount of a soluble organic lubricant comprising at least one carboxylic ester compound characterized by the formula: EQU RO(R.sup.1 O).sub.n C(O)R.sup.2 and R.sup.3 OC(O)R.sup.2
wherein R is a hydrocarbyl group of at least 2 carbon atoms; R.sup.1 is a hydrocarbylene group; R.sup.2 is H, hydrocarbyl, --CF.sub.3, --R.sup.4 CN, --R.sup.4 --NO.sub.2, R.sup.5 OCH(R.sup.6)--; R.sup.3 is a --R.sup.4 CF.sub.3, --R.sup.4 CN or --R.sup.4 NO.sub.2 group; n is an integer from 1 to about 50; R.sup.4 is a hydrocarbylene group; R.sup.5 is H, a lower hydrocarbyl group or R.sup.7 C(O)-where R.sup.7 is a hydrocarbyl group; and R.sup.6 is H or a lower hydrocarbyl group.
Japan Kokai 173,195 (1990) discloses a lubrication oil for a refrigeration compressor which compound is represented by the formula EQU R--O--(AO)--CH.sub.2 CH.sub.2 CN
wherein R is selected from hydrocarbon groups containing 1-12 carbon atoms, cyanoethyl group, and a hydrogen atom; AO is an oxyalkylene group containing 2-18 carbon atoms; and n is an integer from 1-50.
U.S. 4,944,890 relates to the use of polymers of R.sub.f (CH.sub.2).sub.n CH.dbd.CH.sub.2 where R.sub.f may be CF.sub.3 -- to C.sub.10 F.sub.21 -- or higher; and n=0-2 as lubricants in HFC-134a refrigeration systems. U.K. 1,508,349 discloses the use of hydrogenated polyalkylene compounds as lubricants for halogenated alkane refrigerants.
U.S. Pat. Nos. 4,208,293, and 4,439,336 relate to the use of fatty acid amides of diethanolamine, fatty acid esters of diethanolamine, and fatty acid ester-amides of diethanolamine in lubricant oil compositions.
While the foregoing are useful for some refrigerant lubrication needs, there continues to be a need for a synthetic lubricant which provides all of the advantages provided by the present invention.